


Feel It In Your Bones

by KingOfWhisperingHearts



Category: Undertale (Video Game)
Genre: Alternate Universe - Monsters Weren't Sealed Underground, Dadster, F/F, F/M, Gaster has the emotional capacity of a teaspoon, Gender-Neutral Pronouns, Mutual Pining, Other Additional Tags to Be Added, Reader has no defined gender, Reader-Insert, Sans and Papyrus are the cutest skelekids, but hes trying, depends on whether or not I can be patient enough to write it as a slow burn, gaster is the smartest dumbass you know, skeleton family shenanigans, slow burn maybe, you tell Sans puns and it drives Gaster nuts
Language: English
Status: In-Progress
Published: 2018-09-21
Updated: 2019-02-01
Packaged: 2019-07-15 01:45:06
Rating: General Audiences
Warnings: No Archive Warnings Apply
Chapters: 3
Words: 4,415
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/16052870
Author URL: https://archiveofourown.org/users/KingOfWhisperingHearts/pseuds/KingOfWhisperingHearts
Summary: You’ve stuck around for the last 20 years of Gaster’s antics, you might as well stick around for a few more and help him raise two kids.





	1. Chapter 1

**Author's Note:**

> As much as I love sweet/kind Gaster fics, I get a kick out of emotionally constipated asshole Gaster and figured I'd try my hand at writing him that way. I typed this all out right now because my brain wouldn't leave me alone so it's probably super rough, plus this is my first time writing for undertale and it's been awhile since I played the game so forgive me if thing are kinda off

You'd been Gaster’s best friend for nearly twenty years now, ever since he had insulted you in the first grade and you had responded by promptly socking him in the face. Though with how _abrasive_ the skeleton monster could be, you weren't just his best friend but his **only** friend and even then he would deny such a thing, spouting on about how emotional connections were a waste of time and beneath him. It was a speech you'd heard many times and after all these years you had become more than capable of tuning out Gaster when he went on one of his many, many impassioned rants but as things were most people lacked your extremely specific Gaster-based skill set (which consisted of an endless well of patience, the ability to shrug off caustic remarks with relative ease, and a general attitude of simply not giving a flying fuck, among other things) and viewed him as a terrifying and unapproachable individual who would gladly bite the head off of the next person to disturb him. His reputation was admittedly well earned and had played a part in ending many relationships over the years, including his marriage.

The former Mrs. W.D. Gaster had been a fairly cheerful, if somewhat vapid, woman (though when she and Gaster fought she did a complete 180 and was somehow more terrifying than him) who truly loved her son. In the end, both had decided that they, and everyone else around them, would be better off if they separated so with a wave goodbye and a hug for Sans, she had taken off to some corner of the globe to ‘find herself’ and hadn’t been back since. These days the only reason you knew she wasn’t in fact, dead in a ditch somewhere as Gaster so often liked to claim (never in front of Sans though) was because she frequently sent postcards from wherever she was currently staying, often with some trinket or toy. Frankly, you respected her for managing to remain married to Gaster for nearly four years as most couldn’t stand him for more than four minutes though you did wish she’d take the time to actually drop by and visit. Sans didn’t really miss her too much, having only been two when she left, but you still worried that her absence would have a negative effect on him. It had certainly had a negative effect on Gaster who went on a two week long drinking binge after the divorce was finalized that only ended when you, very firmly, reminded him that he had a son to raise and a company to run, which meant you had marched into the bar with Sans on your hip and dragged Gaster back home kicking and screaming to force him to sober up and listen to reason.

Now, three years later, you’re starting to wonder if you’re going to have to stage another intervention as Gaster hasn’t come into the lab for three days and has been ignoring all calls. The second part wasn’t so unusual as he often dismissed any calls from people he found annoying and unimportant but he was also ignoring your calls, which he only ever did when he was doing something he knew you would disapprove of. You wouldn’t be so worried if it weren’t for Sans, who had only just turned five and was still far too young to take care of himself.

Decision made, you snag your keys from the table, taking a brief moment to mourn what was supposed to be your day off before exiting your apartment and hightailing it to your car. You manage to take what is normally a 20-minute drive and cut it down to ten minutes thanks to a combination of, what you’d like to think, is some impressive driving and a number of traffic violations. You pull into the driveway of Gaster’s home, practically leaping out of your car and heading straight for the front door. Cursing under your breath you fumble with your key ring, but before you can locate the spare key Gaster had given you, the door swings open. 

“hey”, Sans greets you casually, tiny hands stuffed into the pocket of his hoodie. You take a moment to look him over, some of the tension leaving you when you realize he’s fine.

“Hey yourself.” You grin at him, crouching down so you’re at eye-level. “Haven’t I been patella-ing you not to answer the door by yourself?” You tap his knee and he giggles, shrugging at you. You straighten up, scooping Sans into one arm as you shut and lock the door behind you. “Where’s your dad kiddo?” Sans, who was more than happy to be carried, had buried his face into your shoulder so when he spoke his words were too muffled for you to understand. “You’re gonna have to say that again Sansy, I didn’t quite catch that. Unless of course you’re”, you paused for dramatic effect, watching as Sans peeked up at you, “giving me the cold shoulder!” You wiggled the shoulder he was leaning on, earning yourself another laugh.

“I saaaaid Dads in the kitchen.” Still laughing, Sans tucked his head back into your shoulder, wrapping his arms around your neck as you walked into the kitchen. Gaster was seated at the table, facing away from you and hunched over with his head in his hands. You padded over, resting a hand on his shoulder. He flinched, startled, before looking up at you, the space beneath his sockets stained a faint purple, a clear indicator that he hadn’t been sleeping much lately. For a moment his expression softened and he leaned into your touch. Then his face quickly twisted into a look of irritation as he pushed back from the table, standing up to loom over you.

“Where have you been?! I haven’t been able to go to the lab for three, three! Days because the _one_ person who I’m supposed to rely on for emergencies can’t be bothered to-”, you lifted a hand to cover his mouth, ignoring his angry sputtering as you withdrew your hand and turned to Sans.

“Listen kiddo why don’t you go to your room and play with your toys while I talk to your dad?” Sans, completely unperturbed by his father’s reaction, gave you a large smile, taking off for his room as soon as you put him down. Turning back to Gaster you crossed your arms, cooly staring him down. “I’ve been gone for the past three days taking care of negotiations for the company that _you_ can’t be bothered to run, negotiations that _you_ specifically told me to handle so _you_ wouldn’t have too.” He opened his mouth to retort and you narrowed your eyes at him. “And may I remind you that _you’re_ the one who's been ignoring all my calls and since _you_ couldn’t be bothered to even shoot me a text I wasn’t even aware that there was an ‘emergency’ until five minutes ago when you told me there was one. Now what, exactly, is this so-called emergency?” Gaster huffed at you, striding forward and hooking his hand into the crook of your elbow, tugging you after him.

“It will be easier to explain if I show you first.” You were annoyed by his manhandling but didn’t voice your displeasure, instead picking up your own pace so he wasn’t dragging you. He lead you to the guest bedroom, stopping just outside the door. He raised a finger to his lips to indicate that you should be quiet and despite your confusion you nodded to show you understood. Gaster gently opened the door and you stepped through, peering into the dark room. As your eyes adjusted, you were able to see that Sans’ old crib had been placed in the corner. You turned back to look at Gatser questioningly and he gestured for you to move forward, so you did, approaching the crib. When you could finally see into the crib, you felt your jaw go slack with surprise.

“Gaster, why do you have a **_baby?!”_**


	2. Chapter 2

You were seated at the kitchen table, the baby cradled in your arms. Your little outburst had woken him up and he had immediately burst into tears, producing more sound than you would have thought possible for such a young child. His wailing had ceased the second you picked him up and he had instead began to babble happily, excitement only increasing as you cooed at him. Now that you were out of the darkened room and you could see him clearly, the resemblance between him and Gaster was undeniable. There was of course the obvious fact that he was a skeleton monster but he also had a skull that was more anatomically similar to that of a human’s much like Gaster’s, and completely unlike Sans whose skull was much more rounded out. You also found yourself thinking of Gaster’s rare smiles whenever the baby stopped chewing on his toy long enough to beam up at you. 

“So tell me again how you ended up with a baby?”

“Well”, Gaster smirked, “when 2 people love each other very much-”

“Shut up, you know that’s not what I meant.” You cut him off, shooting him a glare as he snickered. “What I wanna know is his name and where the other parent is?”

“His name is Papyrus.” Gaster’s face darkened. “And as for the mother, she’s made it _quite_ clear that she no longer wants to be apart of his life. That’s for the best of course, she certainly wasn’t cut out for raising a child and he’s much better off with me than _her_. 

You raised an eyebrow. “How’d you even meet her anyways? You haven’t been on a date in years and you hardly ever leave the lab or house without me or Sans.”

Gaster, in an uncharacteristically nervous gesture, fiddled with the cuffs of his sleeve. “I assume you remember when I attended that science conference?”

“Yeah, you threw a fit when I suggested you go, complaining about how it would be filled with ‘bumbling idiots who thought that wearing a lab coat and splashing together a few chemicals made them scientists.’ You only agreed to go when I promised to watch Sans for the weekend and supervise the lab while you were gone. Even then you still complained up until the moment you walked out the door.” You rolled your eyes at the memory of Gaster making a fuss as he made last minute preparations, pausing just long enough in his complaints to press a gentle kiss to a sleeping Sans’ forehead before resuming his grumbling as he departed.

“Yes, well, the conference wrapped up sooner than anticipated and I decided to indulge in a drink at the hotel bar.” 

You pinched the bridge of your nose with your free hand, already seeing where this story was going. “You’re telling me you got drunk, slept with some woman, knocked her up and, let me guess, just now found out about your second son.” You were careful to control the volume of your voice, mindful of the fact that Papyrus had dozed off.

Gaster huffed, “I wouldn’t put it so crudely but you are correct in assuming that I wasn’t previously aware of Papyrus. If I had been I would have taken custody much, much sooner.” You believed him when he said that, Gaster was often times insensitive, rude, and occasionally even cruel, but he was fiercely protective of Sans, and now that he was aware of Papyrus he would be just as protective of him.

“If it happened at the science conference, that would make him about six months old, why’d she only just now leave him with you?”

“As if I would have any idead about what goes through that simpleton's mind, all that matters now is that that repulsive woman is gone and out of _my_ son’s life.”

“If she disgusts you so much, why’d you sleep with her?”

“As you so kindly pointed out, I was inebriated, and not in my right state of mind, otherwise I would never have given her the time of day, much less sleep with her and, as you said, ‘knock her up.’” Gaster brought his hands up, fingers forming air quotes, and you snorted at his deadpan expression. 

“Guess this means you’ll be taking some time off to get this little guy situated. And don’t even start on how the lab needs you there in order to function, you know just as well as I do that the team is more than capable of holding down the fort while you’re gone. Besides I can always step in if it comes to that.” Gaster opened then shut his mouth, aware that you were correct but unwilling to admit it.

“Fine. But you’re going to help me.” You weren’t surprised by his demand, while your official job was to act as a liaison between Gaster and other people (because any interaction with Gaster without you there as a buffer was an overwhelmingly negative experience), you also frequently found yourself involved in his personal affairs, with tasks like babysitting Sans, picking Sans up from school, and making sure both of them ate at least one decent meal a day being just a few examples of the things you did for them. You didn’t really mind it either, as much as he liked to deny it Gaster _was_ your best friend and Sans was a cute kid so it wasn’t like spending time with the little family was a hassle.

“Yeak ok. Do you have everything Papyrus needs yet or do we need to go shopping?” Gaster, clearly pleased by your easy acceptance, shook his head.

“Besides a few change of clothes, some formula, the stuffed rabbit, and a few other basic items, she didn’t bring anything else when she left him with me and I haven’t had the chance to pick anything else up yet, so he's been using many of Sans' hand me downs.” You stood up, passing the sleeping Papyrus off to Gaster.

“I’ll make a few calls, let everyone know that we’re gonna be taking some time off and then I’ll head to the store. Go ahead and write everything down and I’ll get the list before I leave.”

“Absolutely not! I refuse to let you shop on your own, knowing you you’ll purchase all the wrong brands and if you think I’m going to provide anything less than the very best for my son then you’ll find that you are **sorely** mistaken.”

****

“Whatever you say Gaster.” You put your hands up in a gesture of surrender. “If you’re coming along then why don’t you get Sans’ old car seat out of the garage and get Papyrus situated. After I finish these calls I’ll go get Sans and then we can all go shopping.” Not waiting for his reply you took off, stepping into the hallway where Gaster, Sans, and now Papyrus’ bedrooms were. Pulling out your phone, you were quick to dial the number for Gaster’s secretary. You simply told her that the two of you would be out for some time, without explaining why and after she promised to reschedule any upcoming events you hung up and called in to the lab to let them know that Gaster’s absence would be continuing. Having taken care of that you went and knocked on Sans’ door.

****

“Knock knock.”

****

“who’s there?” Sans called out.

****

“Etch.”

****

“etch who?”

****

“Bless you!” You chuckled as Sans opened the door, smile wide as he laughed at your joke. “C’mon skelekid, put on your shoes, we’re going shopping with your dad and brother.” Sans’ pupils turned to stars and he dashed past you to snag a pair of pink sneakers from the rack by the front door, nearly skidding into the wall. He was quick to jam his feet into them, clumsily tying the laces the way you and Gaster had taught him, jumping to his feet as soon as he finished. “Looks good Sans, I can tell you’ve been practicing.” 

****

Swinging the door open you waited for Sans to step out before following after him and locking the door. Gaster, having come out before you, was currently hunched over, strapping Papyrus into his car seat. Sans had already opened the door on the other side, clambering in and settling into his booster seat. You walked over, helping him buckle up before shutting the back door and opening the one to the driver’s seat. Gaster, having finally adjusted the car seat to his satisfaction, stood up, glaring at you from the other side of the car.

****

“What do you think you’re doing?”

****

“I dunno Gaster, you’re the super genius here, what do _you think_ I’m doing?”

****

His glare intensified. “You’re not driving.”

****

You shrugged, moving to the other side of the car to slide into the passenger seat and Gaster was quick to get into the driver's seat. As you both put on your seatbelts you glance at the two boys in the back. 

****

“Gaster when did you even get a ‘Baby on Board’ sign and why do we need it?” Attached to the backseat window was an obnoxiously yellow sign that you were certain you had never seen Gaster use, not even when Sans was a baby.

****

“I’ve always had that, it’s good to remind all those reckless idiots out on the road of who they’re endangering with their carelessness. It’s all well and good if they want to risk their own life but to jeopardize the safety of children, my children is another matter entirely!” Sensing a rant incoming you tuned him out, turning to stare out the window as Gaster finally started the car. It was going to be a long ride.

****

**Notes for the Chapter:**

> Wow ok so I didn't think I'd be updating so soon but my brain wouldn't let me sleep till I typed all this out so here we are. If I can keep this up the next chapter will probably be out in a few days. If you guys are enjoying the story so far please hit the kudos button or leave a comment, it's super motivating


	3. Chapter 3

**Notes for the Chapter:**

> Sorry for how long its been college has been kinda kicking my ass and I was going through some personal stuff. Honestly I kinda forgot about this fic but BloodQuil's comment reminded me it existed and was very kind so I just finally sat down with my laptop and typed out a chapter real quick, so you can thank them for motivating me to finally post a new chapter

Sans tugged on your hand, anxious to get to the toy aisle. The two of you had separated from Gaster and Papyrus, the former having taken off to the infant section of the store the second he had secured Papyrus to his chest in the baby carrier, trusting you to look after Sans. Sans had, of course, wanted to look at the toys and unable to deny him, especially when he looked at you with puppy eyes, you had immediately given in. You now found yourself watching as Sans flitted from one display to another, fawning over each and every one of them, though you noticed he paid particular attention to the model rocket kits. As he moved on you sidled up to the shelf, discreetly sliding one of the kits into your shopping basket. Satisfied, you trailed after the tiny skeleton. 

Eventually Sans grew bored of wandering through the aisles and the two of you went to join Gaster and Papyrus. You found Gaster debating the merits of different baby bottles, muttering under his breath as he paced up and down the aisle. The other browsing shoppers eyed him warily everytime he forcefully placed a bottle back on the shelf with a derisive snort. His basket was still empty so you were pretty sure he’d been doing this the entire time you’d been shopping. Plucking the bottle he was currently examining out of his hand you tossed it into the basket with two more. Ignoring his spluttering, you waved at Papyrus, smiling as he shrieked wordlessly, tiny arms flailing. Gently pulling on the back of Papyrus’ onesie to check the tag, you made note of the size and turned back around. 

“Sans, buddy, wanna help me pick out all the coolest clothes for Paps?” His eyes lit up and he nodded, allowing you to scoop him up so he could reach the rack. Almost immediately he selected a blue onesie dotted with embroidered stars, a rocketship on the front. 

“What about this one?”

“That one looks super cool skelekid. What do you think Papyrus?” Shifting Sans to rest on your hip, you turned to Gaster who had walked up behind you. Papyrus, from his spot on his father’s chest, squealed when you held up Sans’ pick, reaching out to try and snag it. Shooting Sans a wink and a smile you laughed. “I think he likes it.” You tucked it into Gaster’s basket and continued to riffle through the racks. Gaster also began to go through the clothes, picking outfits that were very plain compared to what you and Sans were choosing, though you gave him a knowing look when he secretly slipped a onesie with little glow in the dark stars in. He returned your look with a glare, though it held no malice in it and was further offset by the faint streak of purple across his cheekbones. By the time the three of you were satisfied, Gaster’s basket was overflowing with clothes and accessories, the other essentials you needed were unceremoniously stuffed into your’s, and Papyrus had fallen asleep. 

The four of you made your way over to the checkout and you grabbed the model rocket, leaving Gaster to keep an eye on Sans while simultaneously folding each article of clothing meticulously before placing them in the shopping bad he had brought with him. The line for the self checkout was empty so you were able to purchase and bag the rocket all before the cashier had finished scanning Gaster’s items. You slid into place at Gaster’s side, dutifully picking up as many bags as you could once he paid. You took one of Sans’ hands in your own, Gaster doing the same on his other side, and exited the store, Papyrus still snoozing away. 

After driving home and receiving yet another lecture on road safety, you and Sans unloaded the car while Gaster prepared a bottle for the now awake Papyrus. Once the two of you were done, you finally showed Sans your purchase, his eyelights forming little stars when he realized what you were holding.

“Is that for me?” 

You tapped your chin thoughtfully, pretending to mull things over. “I dunno, is there any other skelekids around who are super cool and smart around that might like an awesome rocket kit?”

“Paps is really cool! And he’s smart for a babybones”, Sans’ voice was filled with pride and it was clear that he already adored his brother after only knowing him for just a few day. “But, he’s too little for a rocket.”

“Oh well then I guess it is for you after all! Now how about you go put it away in your room for now, wash your hands, and come help me get dinner ready?” Sans nodded eagerly, darting off to his room. You entered the kitchen, striding past Gaster, who was feeding Papyrus, and making your way to the pantry. With practiced ease, you flicked the stove on, placing a put of water on top and sprinkling in some salt and olive oil. After that was set, you searched the fridge for tomato sauce, quickly finding it and setting another pot on the stove to heat it up in. When Sans joined you, the water was heated enough for you to toss in some spaghetti noodles and you allowed Sans to pour in the rest. Even with a stool, he was too short to reach the pot so you gave him a boost, resting him on your hip and watching carefully as he stirred the noodles. In no time at all the spaghetti was done and you had set the table with some more assistance from Sans, who had taken a seat across from his father. After making sure Sans was served, you settled down with your own plate. Papyrus was stubbornly refusing to eat any of the food he was being offered, only relenting when Gaster added sound effects, moving the spoon about as if it were a plane. The scene was painfully adorable and you casually pulled out your phone to take a video. Gaster was far too busy trying to prevent Papyrus from smacking the spoon out of his hand to notice, but Sans looked at you knowingly, giggling when you shot him a wink.

By the time Gaster had finished feeding Papyrus and cleaned up the resulting mess, you had already gone through Sans’ bedtime routine, which included reading him a short book about space that you had given to him last Gyftmas. When you returned to the kitchen, you found Gaster rocking a sleeping Papyrus in his arms, humming softly as he stared out the window into the night. You paused for a second, simply taking in the sight of Gaster looking so at peace, despite his apparent exhaustion. 

“C’mon G, you need to get some rest too. I can tuck Papyrus in.” You held your arms out, indicating that he should hand you the baby. For a moment his mouth twisted into a sneer, as if to reject your offer, but it wavered and you could tell he was considering it. “I’ll be in the guest room too, you won’t have to worry about him tonight.” Finally he gently placed Papyrus into your arms with only some reluctance, the fatigue winning out over his desire to stay up and watch over the infant. 

He took one last moment to smooth his hand over Papyrus’ skull, tired eyes crinkling at the corners as he gave the barest hint of a smile. Then he looked up to you, mouth opening briefly before shutting again, free hand hovering awkwardly in between the two of you before he withdrew it, clearing his throat loudly. He gave you a short nod, ignoring your raised eyebrow. “Goodnight.” Turning on his heel he walked down the hallway to his bedroom, opening the door and hesitating briefly before turning to face you again. “And...thank you.”

You smiled at him. “It’s what friends are for Gaster.”

“Yes of course”, he said, turning back towards his room, sounding strangely melancholic, “friends.” You watched as he shut the door behind him, the click of it closing sounding disproportionately loud in the silence of the hallway. You went back to the guest bedroom, carefully placing Papyrus in his crib and covering him with a blanket before pulling out one of the sets of pajamas that you had stored in the wardrobe. After changing, you settled into the bed, the day’s excitement finally taking its toll on you in the form of a sudden wave of tiredness. As you drifted off, you couldn’t help but wonder what Gaster had really meant.

**Notes for the Chapter:**

> Its just about 3 in the morning where I live and I typed this all up right now so there's probably a lotta mistakes, I'll come back for those later. Not sure when the next chapter will be, partly cause like I said before college is taking up a lot of my time but also because I'm not entirely sure of where I want to take this story. On the bright side though I do have some ideas for a series of undertale oneshots based off of one of my favorite albums and I've got 1/3 of one of them written so keep an eye out for that

**Author's Note:**

> It's super late and I'm super tired so there's probably tons of mistakes, I'll go through and fix em up later. I'm gonna try and scratch out another chapter of this but I'm not sure when that'll be posted cause I just started my freshman year of college and things are pretty hectic right now. Thanks for reading!


End file.
